Feedback control of polishing using optical detection of clearance

ABSTRACT

A method of controlling polishing includes polishing a first substrate having an overlying layer on an underlying layer or layer structure. During polishing, the substrate is monitored with an in-situ monitoring system to generate a sequence of measurements. The measurements are sorted into groups, each group associated with a different zone of a plurality of zones on the substrate. For each zone, a time at which the overlying layer is cleared is determined based on the measurements from the associated group. At least one second adjusted polishing pressure for at least zone is calculated based on a pressure applied in the at least one zone during polishing the substrate, the time for the at least one zone, and the time for another zone. A second substrate is polished using the at least one adjusted polishing pressure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser.No. 61/379,273, filed Sep. 1, 2010, which is incorporated by reference.This application is also a continuation-in-part of U.S. application Ser.No. 12/697,177, filed Jan. 29, 2010, which is a continuation-in-part ofU.S. application Ser. No. 12/267,526, filed on Nov. 7, 2008, each ofwhich is incorporated by reference.

TECHNICAL FIELD

This disclosure relates to using optical monitoring to controlpolishing.

BACKGROUND

An integrated circuit is typically formed on a substrate by thesequential deposition of conductive, semiconductive, or insulativelayers on a silicon wafer. One fabrication step involves depositing afiller layer over a non-planar surface and planarizing the filler layer.For certain applications, the filler layer is planarized until the topsurface of a patterned layer is exposed. A conductive filler layer, forexample, can be deposited on a patterned insulative layer to fill thetrenches or holes in the insulative layer. After planarization, theportions of the conductive layer remaining between the raised pattern ofthe insulative layer form vias, plugs, and lines that provide conductivepaths between thin film circuits on the substrate. For otherapplications, such as oxide polishing, the filler layer is planarizeduntil a predetermined thickness is left over the non planar surface. Inaddition, planarization of the substrate surface is usually required forphotolithography.

Chemical mechanical polishing (CMP) is one accepted method ofplanarization. This planarization method typically requires that thesubstrate be mounted on a carrier or polishing head. The exposed surfaceof the substrate is typically placed against a rotating polishing pad.The carrier head provides a controllable load on the substrate to pushit against the polishing pad. An abrasive polishing slurry is typicallysupplied to the surface of the polishing pad.

One problem in CMP is determining whether the polishing process iscomplete, i.e., whether a substrate layer has been planarized to adesired flatness or thickness, or when a desired amount of material hasbeen removed. Variations in the slurry distribution, the polishing padcondition, the relative speed between the polishing pad and thesubstrate, and the load on the substrate can cause variations in thematerial removal rate. These variations, as well as variations in theinitial thickness of the substrate layer, cause variations in the timeneeded to reach the polishing endpoint. Therefore, the polishingendpoint usually cannot be determined merely as a function of polishingtime.

In some systems, a substrate is optically monitored in-situ duringpolishing, e.g., through a window in the polishing pad. However,existing optical monitoring techniques may not satisfy increasingdemands of semiconductor device manufacturers.

SUMMARY

Due to the variations discussed above, an overlying layer can be clearedfrom different regions of a substrate at different times. For somematerials, optical monitoring does not reliably detect the thickness ofthe layer being polished. However, optical monitoring can usually detectclearance of the overlying layer. A technique to improve uniformity inthe time that at which an overlying layer is cleared (also termed the“clearing time”) from different regions of the substrate is to detectclearance of the overlying layer for different regions of a firstsubstrate, and adjust at least one polishing pressure on a subsequentsecond substrate based on the times that clearance was detected in thefirst substrate.

In one aspect, a method of controlling polishing includes polishing afirst substrate having an overlying layer on an underlying layer orlayer structure, during polishing, directing a light beam onto the firstsubstrate, the light beam reflecting from the first substrate togenerate a reflected light beam, during polishing, generating a sequenceof measurements of intensity of the reflected light beam, sorting themeasurements into groups, each group associated with a different zone ofa plurality of zones on the substrate, for each zone, determining a timeat which the overlying layer is cleared based on the measurements fromthe associated group, calculating at least one second adjusted polishingpressure for at least zone based on a pressure applied in the at leastone zone during polishing the substrate, the time for the at least onezone, and the time for another zone, and polishing a second substrateusing the at least one adjusted polishing pressure.

Implementations can include one or more of the following features. Thelight beam may be a non-polarized light beam. The non-polarized lightbeam may be a laser beam. The non-polarized light beam may be abroadband visible light beam. The overlying layer may be GST, the lightbeam may include an infra-red component, and the measurements ofintensity of the reflected light beam may be measurements of intensityof an infra-red component of the reflected light beam. The overlyinglayer may be a metal, e.g., copper, aluminum, tungsten, tantalum,titanium or cobalt, the light beam may include a red component, and themeasurements of intensity of the reflected light beam may bemeasurements of intensity of the red component of the reflected lightbeam. The zones may be concentric radial zones. Polishing may includepolishing with a carrier head having a plurality of chambers to applyindependently adjustable pressures to the plurality of zones on thesubstrate. During polishing of the first substrate a first chamber ofthe plurality of chambers may apply a first pressure to a first zone ofthe plurality of zones and a second chamber of the plurality of chambersmay apply a second pressure to a second zone of the plurality of zones.Determining a time at which the underlying layer is exposed for eachzone may include determining a first time for a first zone from theplurality of zones and determining a second time for a second zone fromthe plurality of zones. At least one adjusted polishing pressure for thefirst chamber may be calculated based on the first pressure, the firsttime and the second time. The second zone may be an innermost zone or anoutermost zone. Calculating the adjusted pressure P1′ may includecalculating P1′=P1*(T1/T2) wherein P1 is the first pressure, T1 is thefirst time and T2 is the second time. Determining a time at which theunderlying layer is exposed may include determining a time at which thesequence of measurements stabilizes. Determining a time at which thesequence of measurements stabilizes may include determining that a slopeof a trace generated by the sequence of measurements remains within apredetermined range for a predetermined time period.

In another aspect, a method of controlling polishing includes polishinga first substrate having an overlying layer on an underlying layer orlayer structure, during polishing, monitoring the substrate with anin-situ monitoring system to generate a sequence of measurements,sorting the measurements into groups, each group associated with adifferent zone of a plurality of zones on the substrate, for each zone,determining a time at which the overlying layer is cleared based on themeasurements from the associated group, calculating at least one secondadjusted polishing pressure for at least zone based on a pressureapplied in the at least one zone during polishing the substrate, thetime for the at least one zone, and the time for another zone, andpolishing a second substrate using the at least one adjusted polishingpressure.

Implementations can include one or more of the following features. Thein-situ monitoring system may include an optical monitoring system thatdirects a light beam onto the substrate. The in-situ monitoring systemmay include a friction sensor. Determining a time at which theunderlying layer is exposed for each zone may include determining afirst time for a first zone from the plurality of zones and determininga second time for a second zone from the plurality of zones. At leastone adjusted polishing pressure for the first chamber may be calculatedbased on the first pressure, the first time and the second time.Calculating the adjusted pressure P1′ comprises calculatingP1′=P1*(T1/T2) wherein P1 is the first pressure, T1 is the first timeand T2 is the second time.

In another aspect, a computer-readable medium has stored thereoninstructions, which, when executed by a processor, causes the processorto perform operations

Implementations can include one or more of the following potentialadvantages. Within-wafer non-uniformity (WIWNU) can be reduced.Clearance of an overlying layer, e.g., a GST layer or a metal layer, canoccur substantially simultaneously over the surface of the substrate,which can improve polishing throughput. A polishing process can beadjusted to compensate for process drift over consumable life (e.g.polish head, pad, or slurry). Head to head variation in removal profilecan also be improved.

The details of one or more embodiments are set forth in the accompanyingdrawings and the description below. Other aspects, features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 shows a schematic cross-sectional view of a polishing station.

FIG. 2 shows a schematic cross-sectional view of a carrier head.

FIG. 3A shows an overhead view of a substrate on a platen and showslocations where measurements are taken.

FIG. 3B shows concentric zones on a substrate.

FIG. 4 shows a schematic exemplary graph of signal intensity as afunction of time for a substrate being polished that includes a layer ofGST.

FIG. 5 shows a schematic exemplary graph of signal intensities frommultiple regions of a substrate being polished that includes a layer ofGST.

FIG. 6 shows a schematic exemplary graph of signal intensities frommultiple regions of a substrate being polished that includes a layer ofmetal.

FIG. 7 shows a flow chart for a polishing process.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

In some semiconductor chip fabrication processes an overlying layer,e.g., a metal such as copper, tungsten, aluminum, titanium, tantalum orcobalt, a semimetal, or an alloy of metal(s) and/or semimetal(s), e.g.,GeSbTe (a ternary compound of germanium, antimony and tellurium, alsoknown as GST), is deposited over a patterned underlying layer or layerstructure, e.g., a stack of one or more other layers. The one or moreother layers can include layers of dielectric material, e.g., a low-kmaterial and/or a low-k cap material, or of barrier metal, e.g.,tantalum nitride or titanium nitride. Often the overlying layer ispolished until it is “cleared”, i.e., until the top surface of theunderlying layer or layer structure is exposed. Portions of theoverlying layer may be left in trenches, holes, etc., provided by thepattern of the underlying layer or layer structure.

In general, it would be desirable to have the overlying layer clearcompletely (e.g., no discontinuous regions of the overlying layercoating the top surface of the underlying layer or layer structure) andat substantially the same time across the surface of the substrate. Thiscan avoid overpolishing, improve throughput and reduce within-wafernon-uniformity (WIWNU).

A problem with monitoring of polishing of the overlying layer is thatfor some materials, e.g., metals with high reflectivity, optical (e.g.,spectrographic) monitoring of the substrate may not provide usefulinformation regarding the thickness of the overlying layer during bulkpolishing. Without being limited to any particular theory, atwavelengths typically used for optical monitoring the extinctioncoefficient of the material of the overlying layer may be sufficientlyhigh that the reflectivity may not appreciably change as the thicknessis reduced during bulk polishing. As such, optical monitoring may not besuitable for in-situ feedback control of polishing parameters duringbulk polishing of some materials. Although the reflectivity of theoverlying layer can change as it is cleared and the underlying layer isexposed, by the time such a change in optical behavior is detected, itcan be too late for an in-situ adjustment of the polishing rate of thesubstrate being polished to improve the uniformity of clearing timeacross the substrate.

In addition, for some of these same materials, eddy current monitoringis not effective in detecting clearance of the overlying layer. Forexample, barrier metals, such as titanium nitride and tantalum nitride,cannot provide a eddy current good signal due to their intrinsic highresistivity. For other metals, e.g., copper, aluminum and tungsten, theeddy current sensor may not pick up a signal from the discontinuousmetal films remaining on the substrate, and thus detection of clearanceof the overlying layer may be unreliable.

However, for many processes the overlying layer has a differentreflectivity than the underlying layer or layer structure. Without beinglimited to any particular theory, this may be because the overlyinglayer has a different, e.g., higher, extinction coefficient than theunderlying layer. In such cases, the reflectivity or reflected spectrumfrom the substrate should change when the overlying layer clears and theunderlying layer is exposed.

A technique discussed below to improve uniformity in the time that theoverlying layer is cleared is to optically detect clearance of theoverlying layer in multiple different regions of a first substrate, andadjust a pressure on at least one region of a subsequent secondsubstrate based on the times that clearance was detected in the firstsubstrate such that clearance will occur closer to the same time, e.g.,at substantially the same time, than without such an adjustment.

While this technique can particularly address the problem describedabove, it is also generally applicable even if optical monitoring canprovide useful information regarding the thickness of the overlyinglayer during bulk polishing. In this case, the technique can have otheradvantages, such as consistency of process control between differentmaterials, or reduction of computational load (because detection ofclearing may be less computationally intense than determination ofthickness). Thus, the technique can be applicable to othersemitransparent materials, e.g. semitransparent metals, e.g., GST. Inparticular, the technique is if there is a clear change in behavior ofthe reflectance trace as the layer is cleared.

FIG. 1 is a schematic cross-sectional view of a chemical mechanicalpolishing station 150 operable to polish the substrate 10. The polishingstation 150 includes a rotatable disk-shaped platen 24, on which apolishing pad 30 is situated. The platen 24 is operable to rotate aboutan axis 25. For example, a motor (not shown) can turn a drive shaft 27to rotate the platen 24. The polishing pad 30 can be detachably securedto the platen 24, for example, by a layer of adhesive. When worn, thepolishing pad 30 can be detached and replaced. The polishing pad 30 canbe a two-layer polishing pad with an outer polishing layer 32 and asofter backing layer 34.

The polishing station 150 can include a combined slurry/rinse arm 39.During polishing, the arm 39 is operable to dispense slurry 38, e.g., aliquid with abrasive particles. Alternatively, the polishing station 150includes a slurry port operable to dispense slurry onto polishing pad30.

The polishing station 150 also includes the carrier head 80 that isoperable to hold the substrate 10 against the polishing pad 30. Thecarrier head 80 is suspended from a support structure, for example, thecarousel 154, and is connected by a carrier drive shaft 74 to a carrierhead rotation motor 76 so that the carrier head can rotate about an axis71. In addition, the carrier head 80 can oscillate laterally in a radialslot formed in the support structure. In operation, the platen 24 isrotated about its central axis 25, and the carrier head 80 is rotatedabout its central axis 71 and translated laterally across the topsurface of the polishing pad 30.

Referring to FIG. 2, the carrier head 80 can include multiple chambersin order to apply independently controllable pressures to multipleregions, e.g., concentric regions, on the substrate. In oneimplementation, the carrier head 80 includes a housing 302, a baseassembly 304, a gimbal mechanism 306 (which can be considered part ofthe base assembly 304), a loading chamber 308, a retaining ring 310, anda substrate backing assembly 320 which includes a flexible membrane 326that defines multiple independently pressurizable chambers, such as aninner chamber 330, a middle chambers 332, 334, 336, and an outer chamber338. These chambers control the pressure on concentric regions of theflexible membrane, thus providing independent pressure control onconcentric portions of the substrate 10. In some implementations, thecarrier head 80 includes five chambers and a pressure regulator for eachof the chambers. For example, referring to FIG. 3B, the five chambers330, 332, 334, 336, and 338 can control the pressure applied to fiveconcentric zones Z1, Z2, Z3, Z4 and Z5 on the substrate 10.

Returning to FIG. 1, the polishing station 150 also includes an opticalmonitoring system, which can be used to determine a polishing endpointas discussed below. The optical monitoring system includes a lightsource 51 and a light detector 52. Light passes from the light source51, through the polishing pad 30, impinges and is reflected from thesubstrate 10 back through the polishing pad, and travels to the lightdetector 52.

Optical access through the polishing pad 30 is provided by including anaperture (i.e., a hole that runs through the pad) or a solid window 36.The solid window can be secured to the polishing pad 30, although insome implementations the solid window 36 can be supported on the platen24 and project into an aperture in the polishing pad 30. In someimplementations the solid window 36 is secured in the polishing pad 30and is a polyurethane window. The polishing pad 30 is usually placed onthe platen 24 so that the aperture or window overlies an optical head 53situated in a recess 26 in the top surface of the platen 24. The opticalhead 53 consequently has optical access through the aperture or windowto a substrate being polished.

A bifurcated optical cable 58 can be used to transmit the light from thelight source 51 to the window 36 and back from the window 36 to thelight detector 52. The bifurcated optical cable 58 can include a “trunk”58 a and two “branches” 58 b and 58 c.

As mentioned above, the platen 24 includes the recess 26, in which theoptical head 53 is situated. The optical head 53 holds one end of thetrunk 58 a of the bifurcated fiber cable 58, which is configured toconvey light to and from a substrate surface being polished. The opticalhead 53 can include one or more lenses or a window overlying the end ofthe bifurcated fiber cable 58. Alternatively, the optical head 53 canmerely hold the end of the trunk 58 a adjacent the window in thepolishing pad. The optical head 53 can be removed from the recess 26 asrequired, for example, to effect preventive or corrective maintenance.

The platen 24 includes a removable in-situ monitoring module 50. Thein-situ monitoring module 50 can include one or more of the following:the light source 51, the light detector 52, and circuitry for sendingand receiving signals to and from the light source 51 and light detector52. For example, the output of the detector 52 can be a digitalelectronic signal that passes through a rotary coupler, e.g., a slipring, in the drive shaft 27 to a controller 90 for the opticalmonitoring system. Similarly, the light source can be turned on or offin response to control commands in digital electronic signals that passfrom the controller 90 through the rotary coupler to the module 50.

The in-situ monitoring module can also hold the respective ends of thebranch portions 58 b and 58 c of the bifurcated optical fiber cable 58.The light source is operable to transmit light, which is conveyedthrough the branch 58 b and out the end of the trunk 58 a located in theoptical head 53, and which impinges on a substrate being polished. Lightreflected from the substrate is received at the end of the trunk 58 alocated in the optical head 53 and conveyed through the branch 58 c tothe light detector 52.

In some implementations, the bifurcated fiber cable 58 is a bundle ofoptical fibers. The bundle includes a first group of optical fibers anda second group of optical fibers. An optical fiber in the first group isconnected to convey light from the light source 51 to a substratesurface being polished. An optical fiber in the second group isconnected to receive light reflecting from the substrate surface beingpolished and convey the received light to a light detector. The opticalfibers can be arranged so that the optical fibers in the second groupform an X-like shape that is centered on the longitudinal axis of thebifurcated optical fiber (as viewed in a cross section of the bifurcatedfiber cable 58). Alternatively, other arrangements can be implemented.For example, the optical fibers in the second group can form V-likeshapes that are mirror images of each other. A suitable bifurcatedoptical fiber is available from Verity Instruments, Inc. of Carrollton,Tex.

There is usually an optimal distance between the window 36 of thepolishing pad 30 and the end of the trunk 58 a of bifurcated fiber cable58 proximate to the window 36 of the polishing pad 30. The distance canbe empirically determined and is affected by, for example, thereflectivity of the window 36, the shape of the light beam emitted fromthe bifurcated fiber cable, and the distance to the substrate beingmonitored. In some implementations, the bifurcated fiber cable issituated so that the end proximate to the window 36 is as close aspossible to the bottom of the window 36 without actually touching thewindow 36. With this implementation, the polishing station 150 caninclude a mechanism, e.g., as part of the optical head 53, that isoperable to adjust the distance between the end of the bifurcated fibercable 58 and the bottom surface of the polishing pad window 36.Alternatively, the proximate end of the bifurcated fiber cable isembedded in the window 36.

If the overlying layer is GST, then the light source 51 can be selectedto emit light in the near infrared range, e.g., monochromatic light,e.g., light with a wavelength of about 1.3 microns. Alternatively, thelight source 51 can be configured to emit light with a narrow bandwidth,e.g. around 1.3 microns. Alternatively, the light source 51 can beconfigured to emit light with a wide bandwidth in the near infraredrange, e.g., including light around 1.3 microns, and the detector 52 canbe configured to detect light with a narrower bandwidth, e.g. around 1.3microns, or the detector can be a spectrometer configured to useintensity measurements from the near infrared range, e.g. around 1.3microns. In some implementations, the light source 51 emits light havingwavelengths in the 2-5 microns range, suitable for GST thicknessmeasurements. In some implementations, the light source 41 emits lighthaving wavelengths in the 10 micron range, suitable for GST structuralphase measurements.

If the overlying layer is another metal, e.g., copper, tungsten,aluminum, titanium or tantalum, or a barrier metal, e.g., titaniumnitride or tantalum nitride, then the light source 51 can be selected toemit light in the visible range, e.g., red light. The light source canemit monochromatic light, e.g., light with a wavelength between about650-670 nm. Alternatively, the light source 51 can be configured to emitlight with a narrow bandwidth, e.g. around 650-670 nm, or with a widebandwidth in the visible light range. The detector 52 can be configuredto detect a total intensity of the reflected light beam across some orall of the visible light range, e.g., the detector can be a simplephotodiode that operates in the visible light range or be a spectrometerconfigured to sum intensities across a wavelength band, or the detectorcan be configured to detect an intensity of the reflected light beam atsubstantially a single wavelength, e.g., the detector can be aphotodiode that operates at a substantially a single wavelength, or aspectrometer configured to use the intensity measurement of a singlewavelength from the detected spectrum. The detector 52 can be sensitiveto red light, e.g., a wavelength between about 650-670 nm.

The light source 51 and light detector 52 are connected to thecontroller 90 to control their operation and to receive their signals.With respect to control, the controller 90 can, for example, synchronizeactivation of the light source 51 with the rotation of the platen 24. Asshown in FIG. 3A, the controller 90 can cause the light source 51 toemit a series of flashes starting just before and ending just after thesubstrate 10 passes over the in-situ monitoring module. (Each of points701-711 depicted represents a location where light from the in-situmonitoring module impinged and reflected off) Alternatively, thecontroller 90 can cause the light source 51 to emit light continuouslystarting just before and ending just after the substrate 10 passes overthe in-situ monitoring module. Although not shown, each time thesubstrate 10 passes over the monitoring module, the alignment of thesubstrate 10 with the monitoring module can be different than in theprevious pass. Over one rotation of the substrate 10, intensitymeasurements are obtained from different angular locations on thesubstrate 10, as well as different radial locations. That is, someintensity measurements are obtained from locations closer to the centerof the substrate 10 and some are closer to the edge. The controller cansort the intensity measurements from the substrate 10 into groupscorresponding to the concentric radial zones, e.g., by calculating thedistance between the location of the intensity measurement and thecenter of the substrate. The radial zones can corresponding to thedifferent controllable zones on the carrier head 80. For example,referring to FIG. 3B, the intensity measurements can be sorted intogroups corresponding to concentric radial zones Z1, Z2, Z3, Z4 and Z5 onthe substrate 10. Three, four, five, six, seven or more zones can bedefined on the surface of the substrate 10.

With respect to receiving signals, the controller 90 can receive, forexample, a signal with the intensity of light received by the lightdetector 52. The controller 90 can process the signal to when theunderlying layer was exposed, and use this information to adjust thepolishing parameters, e.g., pressure in one of the carrier headchambers, in order to improve polishing uniformity.

Referring to FIG. 4, for a given radial zone, the sequence ofmeasurements from a series of sweeps of the sensor generates anintensity trace 800 which is a function of time or number of platenrotations. As illustrated, for polishing of a GST layer, the intensityof light reflected from the substrate 10 evolves as polishingprogresses, passing through one or more peaks 802 and/or valleys 804,and then stabilizing at a plateau 806. While the intensity trace 800shown in FIG. 4 is merely illustrative, and have many other shapes,intensity traces generated during polishing of GST will have a commonfeature in that after an initial period of variation, the intensitytrace stabilizes at a plateau 806.

Without being limited to any particular theory, as the GST layer isbeing polished, its thickness changes. The change in thickness causes avariation in the interference between the light reflected from thesurface of the GST layer and any underlying layer, resulting invariations in the intensity of the reflected light. However, once thelayer underlying the GST layer is exposed, the signal is primarily dueto reflection from the underlying layer, and the reflected signalintensity stabilizes. By detecting when the intensity trace stabilizes,the controller can determine the time at which the GST layer was clearedand the underlying layer was exposed. Detection of stabilization of theintensity trace can include detecting whether the slope of the traceremains within a predetermined range (near zero slope) for somethreshold time period 810. Detection of stabilization of the intensitytrace can also include detecting whether the magnitude of the traceremains within a range (set relative to the magnitude at the beginningof the time period) for the threshold time period.

As noted above, the intensity measurements from the optical sensor canbe sorted into different radial zones. This permits creation of aseparate intensity trace for each radial zone. For example, as shown inFIG. 5, if the intensity traces are divided into five radial zones, thenfive corresponding traces, e.g., traces 810, 812, 814, 816 and 818, canbe generated.

Referring to FIG. 6, for polishing of an overlying layer that is areflective material, e.g., a metal, such as copper, tungsten, aluminum,titanium or tantalum, the sequence of measurements from a series ofsweeps of the sensor generates multiple intensity traces which ares afunction of time or number of platen rotations. For example, if theintensity traces are divided into five radial zones, then fivecorresponding traces, e.g., traces 910, 912, 914, 916 and 918, can begenerated. In general, for each trace, the intensity remains relativelystable in an initial plateau 922. However, as the overlying layer clearsand the underlying layer is exposed, the intensity trace has a sharpdrop 924. Once the top surface of the underlying layer is completelyexposed, the intensity trace stabilizes in a second plateau 926. Bydetecting when the intensity trace drops and then stabilizes, thecontroller can determine the time at which the reflective overlyinglayer was cleared and the underlying layer was exposed in each region ofthe substrate.

A method of polishing will be explained with reference to FIG. 7. Afirst substrate with an overlying layer is polished using a carrier headwith multiple controllable zones and using a default pressure for eachzone (step 910). For example, if there are five zones, then pressuresP1, P2, P3, P4 and P5 can be applied by the five chambers of the carrierhead to the respective zones Z1, Z2, Z3, Z4 and Z5 (see FIG. 3B) on thesubstrate. During polishing, the overlying layer is monitored in-situusing the optical monitoring system. Intensity measurements from themonitoring system are sorted into groups corresponding to radial zones,and for each zone, the time that the overlying layer is cleared toexpose the underlying layer is calculated based on the measurements fromthe corresponding group (step 920). For example, referring to FIG. 5,five intensity traces 810, 812, 814, 816 and 818, can be generated, withresulting clearance times T1, T2, T3, T4 and T5, for the respectivezones Z1, Z2, Z3, Z4 and Z5 (see FIG. 3B). For example, referring toFIG. 6, five intensity traces 910, 912, 914, 916 and 918, can begenerated, with resulting clearance times T1, T2, T3, T4 and T5, for therespective zones Z1, Z2, Z3, Z4 and Z5 (see FIG. 3B).

Returning to FIG. 7, next, adjusted polishing pressures are calculatedfor at least one of the controllable zones of the carrier head (step930). Pressures can be calculated under a simple Prestonian model toadjust the polishing rates to cause each zone to clear at approximatelythe same time. One zone, e.g., the innermost zone Z5, can be selected asthe reference zone. For each other zone, an adjusted pressure iscalculated by multiplying the default pressure by the ratio of thepolishing time of the zone being adjusted to the polishing time of thereference zone. For example, adjusted pressures P1′, P2′, P3′ and P4′for zones Z1, Z2, Z3, Z4 can be calculated as P1′=P1*(T1/T5),P2′=P2*(T2/T5), P3′=P3*(T3/T5) and P4′=P4*(T4/T5). Alternatively, theoutermost zone, or one of the middles zones, could be selected as thereference zone. A subsequent substrate is then polished with theadjusted polishing pressures (step 940).

Polishing of the subsequent substrate can monitored with the opticalmonitoring system), a new set of clearance times T1, T2, T3, T4 and T5,for the respective zones Z1, Z2, Z3, Z4 and Z5 can be determined (step910), and a new set of adjusted pressures calculated with the previouslycalculated adjusted pressures being used as the new default pressures(step 920), and another substrate polished with the new set of adjustedpressures. More generally, the system can perform an iterative feedbackmethod, in which, for each next substrate to be polished, the clearancetimes and pressures for the prior substrate are used to calculateadjusted pressures for the next substrate. In addition, it is possiblethat adjusted pressures could be calculated based on a weighted runningaverage of clearance times and/or pressures for multiple priorsubstrate, rather than only the immediately prior substrate.

In some implementations, a substrate with an overlying layer is polishedat a first platen for a predetermined amount of time to perform bulkclearing, and then polished at a second platen using the techniquedescribed for FIG. 7. If the overlying layer is a conductive material,then at the first platen, thickness of the layer can be monitored withan eddy current monitoring system. That is, the substrate is polished atthe first platen, the thickness of the conductive layer is monitoredin-situ using the eddy current monitoring system, thickness measurementsfrom the first monitoring system are sorted into groups corresponding toradial zones, and the controller calculates a projected times for eachradial zone for the overlying layer to reach a target thickness. One ormore adjusted pressures are calculated so that the radial zones of thesubstrate will reach the target thickness closer to the same time thanwithout such adjustment, and the adjusted pressures are applied tocomplete polishing of the substrate at the first platen. Then thesubstrate is polished at a second platen, the thickness of the overlyinglayer is monitored in-situ using an optical monitoring system, intensitymeasurements from the second monitoring system are sorted into groupscorresponding to radial zones, the controller detects the times at whichthe intensity traces reach the plateau indicating that overlying layerhas cleared, and one or more adjusted pressures for polishing asubsequent substrate at the second platen are calculated.

Although the discussion above focuses on detection of clearing usingoptical techniques, the process may be applicable to other in-situmonitoring techniques that can detect clearance of the overlying layer,such as local surface friction sensing (e.g., as described in U.S. Pat.No. 7,513,818, incorporated by reference), or eddy current monitoring(assuming that accuracy of the eddy current monitoring technique todetect clearance is eventually improved).

Implementations and all of the functional operations described in thisspecification can be implemented in digital electronic circuitry, or incomputer software, firmware, or hardware, including the structural meansdisclosed in this specification and structural equivalents thereof, orin combinations of them. Implementations described herein can beimplemented as one or more computer program products, i.e., one or morecomputer programs tangibly embodied in an information carrier, e.g., ina machine readable storage device or matters capable of effecting apropagated signal, for execution by, or to control the operation of,data processing apparatus, e.g., a programmable processor, a computer,or multiple processors or computers.

A computer program (also known as a program, software, softwareapplication, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile. A program can be stored in a portion of a file that holds otherprograms or data, in a single file dedicated to the program in question,or in multiple coordinated files (e.g., files that store one or moremodules, sub programs, or portions of code). A computer program can bedeployed to be executed on one computer or on multiple computers at onesite or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

The above described polishing apparatus and methods can be applied in avariety of polishing systems. Either the polishing pad, or the carrierhead, or both can move to provide relative motion between the polishingsurface and the wafer. For example, the platen may orbit rather thanrotate. The polishing pad can be a circular (or some other shape) padsecured to the platen. Some aspects of the endpoint detection system maybe applicable to linear polishing systems (e.g., where the polishing padis a continuous or a reel-to-reel belt that moves linearly). Thepolishing layer can be a standard (for example, polyurethane with orwithout fillers) polishing material, a soft material, or afixed-abrasive material. Terms of relative positioning are used; itshould be understood that the polishing surface and wafer can be held ina vertical orientation or some other orientations.

The term “data processing apparatus” encompasses all apparatus, devices,and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them.

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices.

Computer readable media suitable for storing computer programinstructions and data include all forms of non volatile memory, mediaand memory devices, including by way of example semiconductor memorydevices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks,e.g., internal hard disks or removable disks; magneto optical disks; andCD ROM and DVD-ROM disks. The processor and the memory can besupplemented by, or incorporated in, special purpose logic circuitry.

To provide for interaction with a user, embodiments of the subjectmatter described in this specification can be implemented on a computerhaving a display device, e.g., a CRT (cathode ray tube) or LCD (liquidcrystal display) monitor, for displaying information to the user and akeyboard and a pointing device, e.g., a mouse or a trackball, by whichthe user can provide input to the computer. Other kinds of devices canbe used to provide for interaction with a user as well; for example,feedback provided to the user can be any form of sensory feedback, e.g.,visual feedback, auditory feedback, or tactile feedback; and input fromthe user can be received in any form, including acoustic, speech, ortactile input.

While this specification contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this specification in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingmay be advantageous. Moreover, the separation of various systemcomponents in the embodiments described above should not be understoodas requiring such separation in all embodiments, and it should beunderstood that the described program components and systems cangenerally be integrated together in a single software product orpackaged into multiple software products.

What is claimed is:
 1. A method of controlling polishing, comprising:polishing a first substrate having an overlying layer on an underlyinglayer or layer structure; during polishing of the first substrate,directing a light beam onto the first substrate, the light beamreflecting from the first substrate to generate a reflected light beam;during polishing of the first substrate, generating measurements overtime of intensity of the reflected light beam; sorting the measurementsinto groups, each group associated with a different zone of a pluralityof zones on the first substrate, the sorting generating a sequence ofmeasurements for each zone; for each zone, determining based on thesequence of measurements for the zone including measurements that occuron or after the overlying layer in the zone is cleared, a time at whichthe overlying layer in the zone is cleared such that a top surface ofthe underlying layer or layer structure is exposed in the zone andstoring the time at which the overlying layer in the zone is cleared;after the overlying layer is cleared such that the top surface of theunderlying layer or layer structure is exposed, calculating at least oneadjusted polishing pressure for at least zone of the plurality of zonesbased on a pressure applied in the at least one zone during polishingthe first substrate, the time at which the overlying layer is clearedfor the at least one zone, and the time at which the overlying layer iscleared for another zone of the plurality of zones; and polishing asecond substrate using the at least one adjusted polishing pressure. 2.The method of claim 1, wherein the light beam is a non-polarized lightbeam.
 3. The method of claim 2, wherein the non-polarized light beam isa laser beam.
 4. The method of claim 2, wherein the non-polarized lightbeam comprises a broadband visible light beam.
 5. The method of claim 1,wherein the overlying layer comprises GST.
 6. The method of claim 5,wherein the light beam includes an infra-red component and themeasurements of intensity of the reflected light beam are measurementsof intensity of an infra-red component of the reflected light beam. 7.The method of claim 1, wherein the overlying layer is a metal.
 8. Themethod of claim 1, wherein the overlying layer is copper, aluminum,tungsten, tantalum, titanium or cobalt.
 9. The method of claim 8,wherein the light beam includes a visible red component and themeasurements of intensity of the reflected light beam are measurementsof intensity of the visible red component of the reflected light beam.10. The method of claim 9, wherein polishing comprises polishing with acarrier head having a plurality of chambers to apply independentlyadjustable pressures to the plurality of zones on the substrate.
 11. Themethod of claim 10, wherein during polishing of the first substrate afirst chamber of the plurality of chambers applies a first pressure to afirst zone of the plurality of zones and a second chamber of theplurality of chambers applies a second pressure to a second zone of theplurality of zones.
 12. The method of claim 1, wherein the zonescomprise concentric radial zones.
 13. A method of controlling polishing,comprising: polishing a first substrate having an overlying layer on anunderlying layer or layer structure; during polishing of the firstsubstrate, monitoring the first substrate with an in-situ monitoringsystem to generate measurements over time; sorting the measurements overtime into groups, each group associated with a different zone of aplurality of zones on the first substrate, the sorting generating asequence of measurements for each zone; for each zone, determining basedon the sequence of measurements for the zone including measurements thatoccur on or after the overlying layer in the zone is cleared, a time atwhich the overlying layer in the zone is cleared such that a top surfaceof the underlying layer or layer structure is exposed in the zone andstoring the time at which the overlying layer in the zone is cleared;after the overlying layer is cleared such that the top surface of theunderlying layer or layer structure is exposed, calculating at least oneadjusted polishing pressure for at least zone of the plurality of zonesbased on a pressure applied in the at least one zone during polishingthe first substrate, the time at which the overlying layer is clearedfor the at least one zone, and the time at which the overlying layer iscleared for another zone of the plurality of zones; and polishing asecond substrate using the at least one adjusted polishing pressure. 14.The method of claim 13, wherein the in-situ monitoring system comprisesan optical monitoring system that directs a light beam onto thesubstrate.
 15. The method of claim 13, wherein the in-situ monitoringsystem comprises a friction sensor.
 16. The method of claim 15, furthercomprising calculating at least one adjusted polishing pressure for thefirst chamber based on the first pressure, the first time and the secondtime.
 17. The method of claim 16, wherein calculating the adjustedpressure P1′ comprises calculating P1′=P1*(T1/T2) wherein P1 is thefirst pressure, T1 is the first time and T2 is the second time.
 18. Themethod of claim 13, wherein determining a time at which the underlyinglayer is exposed for each zone comprises determining a first time for afirst zone from the plurality of zones and determining a second time fora second zone from the plurality of zones.
 19. A computer programproduct, tangibly embodied in a non-transitory computer readable media,comprising instructions for causing a processor to: during polishing ofa first substrate, receive measurements over time of the first substratefrom an in-situ monitoring system; sort the measurements over time intogroups, each group associated with a different zone of a plurality ofzones on the first substrate so as to generate a sequence ofmeasurements for each zone; for each zone, determine a time at which theoverlying layer is cleared based on the measurements from the associatedgroup; for each zone, determine based on the sequence of measurementsfor the zone including measurements that occur on or after the overlyinglayer in the zone is cleared, a time at which the overlying layer in thezone is cleared such that a top surface of the underlying layer or layerstructure is exposed in the zone and storing the time at which theoverlying layer in the zone is cleared; after the overlying layer iscleared such that the top surface of the underlying layer or layerstructure is exposed, calculate at least one adjusted polishing pressurefor at least zone of the plurality of zones based on a pressure appliedin the at least one zone during polishing the first substrate, the timeat which the overlying layer is cleared for the at least one zone, andthe time at which the overlying layer is cleared for another zone of theplurality of zones; and cause the polishing system to polish a secondsubstrate using the at least one adjusted polishing pressure.
 20. Thecomputer program product of claim 19, wherein the instructions todetermine a time at which the underlying layer is exposed for each zonecomprise instructions to determine a first time for a first zone fromthe plurality of zones and to determine a second time for a second zonefrom the plurality of zones.
 21. The computer program product of claim20, further comprising instructions to calculate at least one adjustedpolishing pressure for a first chamber of a carrier head based on afirst pressure, the first time and the second time.
 22. The computerprogram product of claim 21, wherein the second zone comprises aninnermost zone on the substrate.
 23. The computer program product ofclaim 21, wherein the second zone comprises an outermost zone on thesubstrate.
 24. The computer program product of claim 21, wherein theinstructions to calculate the adjusted pressure P1′ compriseinstructions to calculate P1′=P1*(T1/T2) wherein P1 is the firstpressure, T1 is the first time and T2 is the second time.
 25. Thecomputer program product of claim 19, wherein the instructions todetermine a time at which the overlying layer in the zone is clearedcomprise instructions to determine a time at which the sequence ofmeasurements for the zone stabilizes.
 26. The computer program productof claim 25, wherein the instructions to determine a time at which thesequence of measurements stabilizes includes instructions to determinethat a slope of a trace generated by the sequence of measurementsremains within a predetermined range for a predetermined time period.